Some battery technologies are very efficient. I think that Li Ion is
>90% efficient. Wikipedia gives Li Poly efficiency at 99%. Capacitors
can be just as good, but it depends on the charging technique. If you
charge a capacitor through a resistor from a voltage source, the
efficiency cannot be better than 50%. You really need to charge them
using a switching-type power supply to get close to the optimal
efficiency.

The major advantage of supercaps as I see it is their very long cycle
life. They can routinely do several million full charge/discharge
cycles. Even the best battery technologies in this regard can only do
perhaps 10 or 20 thousand full charge/discharge cycles.

The best supercaps out there have roughly 10% of the energy density
per volume of lead acid batteries. Energy density per mass is probably
more like 20% of lead acid due to the lower density of the capacitor.
Supercaps are way more expensive than any typical kind of battery on a
per energy basis (energy for 1 full charge) but taking into account
the cycle life, the cost per total energy throughput is probably a
little less for supercaps than for batteries.

Sean


On Wed, Nov 18, 2009 at 9:34 AM, sergio masci <smplx@allotrope.net> wrote:
>
>
> On Tue, 17 Nov 2009, William "Chops" Westfield wrote:
>
>> > supercaps
>>
>> Supercaps have a very bad discharge curve for powering nearly anything.
>> Almost any battery is MUCH better...
>
> Yes but... I can't remember the numbers (maybe someone could enlighten us)
> but batteries are pretty bad at converting electrical energy into chemical
> energy. I seem to remember lead acid were the best and these were only 60%
> efficient. Anyway, supercaps seem to be much more efficent on the input
> side. They are not very good at long term storage (self discharge is
> abismal compared to even NiMH) but they can be cycled 1000's of times
> more than even good NiMH batteries and charging is VERY simple compared to
> a battery.
>
> What about adding a simple boost regulator to a supercap. Yes the output
> would be much less efficient than a battery but wouldn't it be a case of
> swings and roundabouts (poor input battery vs poor output supercap +
> boost).
>
> Regards
> Sergio Masci
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